1. Field of the Invention
The present invention relates to a surface acoustic wave device having a quartz substrate.
2. Description of the Related Art
There has been a great demand for filters which have a middle frequency bandwidth for use in mobile communication equipment and other consumer electronic devices. In general, a bandwidth of a surface acoustic wave device is proportional to a square of an electromechanical coefficient K (K.sup.2) of the device.
A surface acoustic wave device having a quartz substrate is desirable because a temperature coefficient of group delay time (TCD) thereof is nearly zero. However, quartz substrates essentially have a low K.sup.2 value, and a band width of a surface acoustic wave device including a quartz substrate is therefore narrow. More specifically, a component including a plurality of conventional surface acoustic wave devices each having an interdigital transducer (IDT) and disposed on a conventional quartz substrate has a K.sup.2 value of about 0.14%.
Substrates made of materials such as LiTaO.sub.3 have large values of K.sup.2, but the bandwidth of a surface acoustic wave device having such a substrate is too wide to satisfy the above-described demand, since the K.sup.2 value is too large. In addition, the substrates made of LiTaO.sub.3 and the like have a high TCD value, which causes the characteristics to change greatly depending on temperature.
To solve these problems, a surface acoustic wave device having an enhanced K.sup.2 value has been proposed. This surface acoustic wave device is produced by forming a piezoelectric thin film on a quartz substrate and forming an interdigital transducer on the piezoelectric thin film.
This conventional surface acoustic wave device 110 will be now described with reference to FIGS. 10 and 11. FIG. 10 is a schematic plan view of the conventional surface acoustic wave device 110. FIG. 11 is a cross-section taken along line W--W of FIG. 10.
As shown in FIGS. 10 and 11, the conventional surface acoustic wave device 110 is constructed by forming a piezoelectric thin film 112 such as a ZnO film and the like on a quartz substrate 111. On the piezoelectric thin film 112, an interdigital transducer 113 having a pair of comb-shaped electrodes which are interdigitated with each other is formed of Al. A pair of reflectors 114 are provided on opposite sides of the interdigital transducer 113. The reflectors 114 are provided to reflect surface acoustic waves propagated thereto and to confine the energy of the surface acoustic waves. Thus, the surface acoustic wave device 110 functions as a resonator. One or a plurality of the surface acoustic devices are used to form a band-pass filter. The K.sup.2 value of the surface acoustic wave device 110 can be enhanced by use of the quartz substrate 111 having a cut angle and a propagation direction selected to achieve a negative TCD value. By using the piezoelectric thin film 112 which has a positive TCD value, the TCD values of the quartz substrate 111 and the piezoelectric thin film 112 can be canceled. Accordingly, the TCD value of the surface acoustic wave device 110 can be adjusted to be close to zero.
The K.sup.2 value of the surface acoustic wave device 110 can be enhanced by providing a short circuiting electrode made of Al or the like, between the quartz substrate 111 and the piezoelectric thin film 112.
Thus, the conventional surface acoustic wave device 110 has a desired K.sup.2 value of about 1% in addition to a TCD value of about zero, thereby realizing a surface acoustic wave device having a middle bandwidth.
Although the conventional surface acoustic wave device 110 has successfully realized a middle bandwidth as explained above, there is another major problem associated with the conventional surface acoustic wave device 110. That is, the conventional surface acoustic wave device 110 has a small impedance ratio Za/Zr. This small impedance ratio presents a problem in that, when the surface acoustic wave device 110 is used as a resonator, the surface acoustic wave device 110 cannot oscillate easily. Moreover, when a filter is constructed using the surface acoustic wave device 110, the filter experiences the disadvantages of a large insertion loss and gradual frequency characteristics at the band ends.